The invention relates in general to the tuning of a radio receiver to a certain frequency and in particular to automatic tuning in order to ensure the best possible radio signal quality.
Field of the Invention
Reception of a radio transmission suffers from spurious effects caused by the widening and shifting of the frequency band of the received signal, other radio transmissions at the same and adjacent frequencies and other electromagnetic radiation sources. In the receiver, the spurious effects are seen as noise and signal distortion. The problem is made more difficult by the fact that the spurious effects usually vary in time, especially in the case of mobile terminals. In addition to external spurious effects, also internal factors in the receiver, such as component aging, manufacturing tolerances, temperature drift, intermodulation frequencies, etc. affect the need for frequency tuning.
Description of the Prior Art
It is known a prior art method called automatic frequency control (AFC), where the receiver includes a variable missing frequency oscillator, a measurement circuit for measuring the received signal power, and a feedback loop which controls the oscillator so that mixing to the intermediate frequency is always performed using the mixing frequency that produces the highest received signal power. The disadvantage of this method is that the highest received signal power does not always correspond to the best signal quality e.g. in a situation where the receiver picks up an unwanted signal occurring at the same or nearly the same frequency as the desired signal. The turned frequency is such that the attenuation of the intermediate frequency signal path is as low as possible. If the transmission curves of the filters are not symmetrical (the attenuation minimum at the middle of the pass band), the intermediate frequency will be detuned even if the receiver received nothing else than the desired signal. Tolerances of filters and their control circuits may cause considerable detunig so that the received signal becomes distorted and its quality deteriorates.
Another known method is to use a frequency synthesizer, or an accurately tuned mixing oscillator which produces a mixing frequency that is a multiple of a fundamental frequency produced by a stable crystal oscillator. As the mixing frequency remains accurately the same, the receive filter can be made very narrowband so that the influence of spurious effects propagating as other frequences is reduced. The disadvantage of this method is that if the transmitter or receiver or a factor reflecting the radio signal on the path moves, the frequency of the desired signal will not remain constant but, instead, will change, which means that it may drift outside the narrow pass band of the filter. Furthermore, the tolerances of the filters and matching circuits may cause the signal path attenuation not be at its minimum at the nominal frequency. Then the signal-to-noise ratios will suffer, ie. the signal quality will get worse. The method is also susceptible to changes in component characteristics that occur in the course of time.
A prior art method is called automatic gain control (AGC), where a signal coming to a receiver is attenuated in the pre-stage of the device if the received signal level is too high. In the AGC, the tuning of the receiver is not changed. For a double super-heterodyne receiver, which has two intermediate frequencies, it is also known to detune the first mixing frequency obtained from the local oscillator so that the first intermediate frequency signal becomes offset. The second mixing frequency from the local oscillator is also detuned, however in such a manner that the second intermediate frequency signal moves, with respect to the nominal frequency, to the direction opposite to that of the first intermediate frequency signal. Then the effective bandwidth across the intermediate frequency stages becomes narrower. Narrowing the widening of the band may be based on the received signal level or on the signal-to-noise ratio. However, adjustment of the bandwidth and noise detection require separate components that increase manufacturing costs.
The method is known from the Finnish patent application FI 962509, wherein a receiver decodes a digital signal and measures the frequency of occurrence of bit errors in the signal. The receiver turns the oscillator frequency used in the down-conversion such that the bit error ratio (BER) is as low as possible. However, this method is relatively slow since the receiver examines several parallel frequencies and it has to demodulate and decode at each frequency a whole frame or other data structure so as to be able to determine, on the basis of the CRC (cyclic redundancy check) code or a corresponding part of said data structure, whether bit errors occurred in the signal on the transmission path.
Objects of the Invention
An object of this invention is to provide a method by means of which a receiver can automatically tune itself to the correct frequency in a quick and easily realizable manner. Another object of the invention is to provide a radio receiver which can apply the method according to the invention and which has no need for expensive special components.
The objects of the invention are achieved by monitoring how the receiver stays locked to the clock pulse of the received signal and by tuning the receiver to the middle of the frequency band where locking is successful.